Redesign Python AI engine (#5991)

# Description of Changes
Redesign the Python AI engine to be properly agentic and make use of
`pydantic-ai` instead of `langchain` for correctness and ergonomics.
This should be a good foundation for us to build our AI engine on going
forwards.

engine/src/stirling/contracts/__init__.py

@@ -0,0 +1,74 @@
+from .agent_drafts import (
+    AgentDraft,
+    AgentDraftRequest,
+    AgentDraftResponse,
+    AgentDraftWorkflowResponse,
+    AgentRevisionRequest,
+    AgentRevisionResponse,
+    AgentRevisionWorkflowResponse,
+)
+from .agent_specs import AgentSpec, AgentSpecStep, AiToolAgentStep
+from .common import ConversationMessage, PdfTextSelection, ToolOperationStep
+from .execution import (
+    AgentExecutionRequest,
+    CannotContinueExecutionAction,
+    CompletedExecutionAction,
+    ExecutionContext,
+    ExecutionStepResult,
+    NextExecutionAction,
+    ToolCallExecutionAction,
+)
+from .health import HealthResponse
+from .orchestrator import OrchestratorRequest, OrchestratorResponse, SupportedCapability, UnsupportedCapabilityResponse
+from .pdf_edit import (
+    EditCannotDoResponse,
+    EditClarificationRequest,
+    EditPlanResponse,
+    PdfEditRequest,
+    PdfEditResponse,
+)
+from .pdf_questions import (
+    PdfQuestionAnswerResponse,
+    PdfQuestionNeedTextResponse,
+    PdfQuestionNotFoundResponse,
+    PdfQuestionRequest,
+    PdfQuestionResponse,
+)
+
+__all__ = [
+    "AgentDraft",
+    "AgentDraftRequest",
+    "AgentDraftResponse",
+    "AgentDraftWorkflowResponse",
+    "AgentExecutionRequest",
+    "AgentRevisionRequest",
+    "AgentRevisionResponse",
+    "AgentRevisionWorkflowResponse",
+    "AgentSpec",
+    "AgentSpecStep",
+    "AiToolAgentStep",
+    "CannotContinueExecutionAction",
+    "ConversationMessage",
+    "CompletedExecutionAction",
+    "EditCannotDoResponse",
+    "EditClarificationRequest",
+    "EditPlanResponse",
+    "ExecutionContext",
+    "ExecutionStepResult",
+    "HealthResponse",
+    "NextExecutionAction",
+    "OrchestratorRequest",
+    "OrchestratorResponse",
+    "PdfEditRequest",
+    "PdfEditResponse",
+    "PdfQuestionAnswerResponse",
+    "PdfQuestionNotFoundResponse",
+    "PdfQuestionNeedTextResponse",
+    "PdfQuestionRequest",
+    "PdfQuestionResponse",
+    "PdfTextSelection",
+    "SupportedCapability",
+    "ToolOperationStep",
+    "ToolCallExecutionAction",
+    "UnsupportedCapabilityResponse",
+]